Embodiments of the present specification relate generally to diagnostic imaging, and more particularly to methods and systems for improved classification of constituent materials in a magnetic resonance (MR) image.
Magnetic resonance imaging (MRI) provides high-quality images with excellent soft-tissue contrast for use in diagnosis and/or treatment of a patient. Particularly, MRI may be used in conjunction with one or more other imaging modalities to provide complementary diagnostic information for use in studying biochemical processes in greater detail. For example, MRI may be used to complement molecular information offered by a positron emission tomography (PET) system for tracking biomarkers with higher sensitivity.
Typically, PET imaging entails photon-electron interactions that may result in attenuation of emitted photons, which in turn, leads to degraded image quality and inaccurate PET quantitation. Use of MRI data aids in correction of PET attenuation values, thus providing more accurate clinical information. Particularly, the PET attenuation values may be corrected using MRI data that is accurately classified into corresponding constituent materials, such as specific tissues, air, fat, water, bone, metal, and/or other materials.
Conventional MRI generates positive response signals having different signal intensities or brightness for portions of an imaged region that includes soft tissues, water, and/or fat. For example, response signals received from liver tissues may have a particular signal intensity that is different from a signal intensity of response signals received from stomach tissues. A difference in signal intensities, thus, may be used to distinctly classify constituent materials in the imaged region.
However, conventional MRI fails to generate response signals having such distinctive signal intensities when imaging regions that are devoid of water or fat molecules. For example, conventional MRI pulse sequences generate indistinct dark signals when imaging regions including air, bone, and/or metal even though these constituent materials have substantially different attenuation values. Moreover, MRI of regions proximal to metal objects such as pacemakers or dental fillings may cause excessive heating and/or image artifacts due to large susceptibility variations between metal and surrounding tissue. Specifically, presence of metals in an imaged region may cause significant resonant frequency changes during MRI, thereby resulting in substantial signal loss, failure of fat suppression, geometric distortion, and bright pile-up artifacts.
Accordingly, certain conventional MRI systems provide classification methods that attempt to differentiate regions including air from bone structures and metal based on differences in signal intensity, relaxivity, chemical shifts, and/or image gradient information. Certain other classification methods entail magnetic field mapping around metal implants using an asymmetric spin-echo MRI sequence, ultra-short echo-time (UTE) and/or zero-echo time (ZTE) MRI pulse sequences to resolve ambiguity between different constituent materials.
However, even such conventional classification methods fail to mitigate inaccuracies in determined diagnostic information resulting from large signal losses caused by presence of metal objects in the imaged region. Specifically, magnetic susceptibility artifacts that arise due to presence of metal, for example, in the dental fillings or in hip-joint implants distort the diagnostic information derived from MR images. The distorted MR image information fails to aid in accurate and efficient classification of imaged regions in and/or near air, bones and/or metal. The inefficient classification, in turn, precludes use of the MR image information for accurate estimation of PET attenuation values and/or any other biochemical investigation. Magnetic susceptibility artifacts, thus, render conventional MRI unsuitable for a variety of clinical applications.